A pressure- and temperature-swing process utilizing oxygen carriers in solution to sorb and desorb oxygen is known. See U.S. Pat. No. 4,451,270, where broad classes of oxygen carriers in solution with axial bases are described; included in those carriers disclosed are Schiff base metal complexes of alkyl-substituted N-N'-bis(salicylidene)tetramethylethylenediamine, commonly referred to as "saltmen." European Patent Application Publication No. 0 304 818 discloses an oxygen separation membrane comprising a permselective film formed by an in situ interfacial polymerization reaction of Co(salen) with certain polymers. Two of the principal problems which interfere with the proper functioning of saltmen complexes as practical oxygen carriers is their tendency to form mu-peroxo dimers, which renders them inactive as carriers, and to bind oxygen irreversibly, which renders them incapable of regeneration. Another drawback of such saltmen complexes as oxygen carriers is that many of them go into solution only at undesirably high temperatures, rendering their use in a pressure- or temperature-swing sorption/desorption process impractical. Still another drawback of such complexes as carriers has been their low solubility in low-polarity solvents, which tends to limit the rate of oxygen production to nonfeasible, low levels, and does not permit the exclusion of water from the solvent or solvent mixture (water deactivates carriers directly and by the formation of carbonic acid with the carbon dioxide in air). A still further problem with such complexes as carriers has been that of ascertaining those with an oxygen-binding equilibrium constant that is sufficiently high to bind or sorb oxygen at a commercially feasible temperature and oxygen pressure, yet sufficiently low to desorb oxygen, the sorption and desorption taking place under conditions of moderate pressure and temperature.
The foregoing shortcomings of saltmen complexes as oxygen carriers are addressed and resolved by the present invention, which is summarized and described in detail below.